Chances are slim that congressional tax panels can come up with a revenue plan for a new five-year farm bill on Friday, according to aides.

Senate Finance Committee Chairman Max Baucus , D-Mont., and House Ways and Means Committee Chairman Charles B. Rangel , D-N.Y., had said earlier this week that they would try to strike a deal before this weekend, but doing so has proved more challenging than expected.

Mixed messages from the Bush administration on what kind of new taxes and other revenue-raisers it would accept have complicated the talks.

Even though it’s unlikely the Friday deadline will be met, a Baucus aide said he’s still hard at work, “fully exploring the possibility of an agreement that will be good for farmers and that can become law.”

It’s unclear when other farm bill negotiators will see a new financing plan, but Sen. Kent Conrad , D-N.D., a senior member of the Finance Committee, said he hoped that would be possible Monday.

The farm bill budget has been a constant source of frustration for lawmakers writing the reauthorization. They have about $280 billion over five years to work with, but are under substantial pressure to provide more than that for nutrition programs, crop subsidies, conservation and renewable energy programs. Under pay-as-you-go budget rules, any spending over that number must be offset by new taxes or spending cuts.

Both the House and Senate bills contained controversial tax packages that drew veto threats from the White House. Now the negotiators are trying to come up with offsets that both chambers — and President Bush — will accept.